DENDROECOLOGICAL ANALYSIS OF FIR (ABIES-ALBA MILL) AND SPRUCE (PICEA-ABIES (L) KARST) AT DIFFERENT STAGES OF DEFOLIATION IN THE FOREST OF VIGODI-DI-TON (TRENTO, ITALY)

Variations in radial growth of 2 Abies alba populations and one Picea abies population from the Trentino (fig1) have been studied in relation to the present state of tree crowns (table I) Calculation of statistical models (fig 2-4) taking into account age trends (ARMA modelling) and tree ring-climate relationships (response function) over 2 successive periods makes the quantification of growth rate changes possible. Similar growth trends (fig 5,6) were identified for the 3 populations until the 1960s, with differences occurring thereafter. After 2 critical years, 1960 and 1970, the now dying trees (defoliation classes 2 and 3) underwent a loss of vitality, whereas the now healthy trees (defoliation classes 0 and 1) rapidly resumed their growth potential. Response functions established for each population and each defoliation class over 4 successive periods at 10 year intervals reveal a gradual decrease in tree sensitivity to climate from the years 1960 and 1970 onwards (figs 7-9). In parallel, the theoretical growth reconstructed for the period 1960-1987, on the basis of the ARMA model calculated for the previous period 1927-1959 and compared with the actual growth, shows that growth deficiency is directly correlated with the present degree of defoliation (fig 10). The present degree of defoliation appears to be the consequence of a loss of vitality that has been initiated by inimical climatic conditions. This sensitivity of trees to climate varies from one tree to another, depending on genetic inheritance (Abies is more affected than Picea) and site conditions